Question

If cardinality of (A U B) = cardinality of A+ cardinality of B. This means ____________

(a) A is a subset of B

(b) B is a subset of A

(c) A and B are disjoint

(d) None of the mentioned

I had been asked this question in a job interview.

This is a very interesting question from Cardinality of Sets topic in portion Basic Structures: Sets, Functions, Sequences, Sums and Matrices of Discrete Mathematics

Answer 1

Right choice is (c) A and B are disjoint

For explanation I would say: Thus if the cardinality of (A U B) = cardinality of A+ cardinality of B, it means they don’t have any element in common, n(A∩B) = 0.